Various shields have recently been developed for the purpose of preventing radiation exposures resulting from leading-edge radiotherapeutics, plant sites making use of atomic power such as atomic power generation sites, and radioactive waste discharged from them; however, there is still much left to be desired in terms of the ability of radiation-shielding materials to shield off radiations, materials themselves, production processes, etc.
Radiations are largely broken down to particle beams such as α-rays, β-rays and neutron beams and electromagnetic waves such as γ-rays and X-rays. Alpha rays are less penetrating so that they can be shielded off with a single one paper sheet. Beta beams, too, can be shielded off with a few mm-thick aluminum foil. However, γ-rays are more penetrating, and in order to obtain a 90% shielding rate concrete will have to have a thickness of 29 cm and even lead will have to have a thickness of 2.5 cm. Neutron beams are much more penetrating, and they cannot be shielded off without hydrogen atoms contained in water or thick concrete walls.
In most cases, heavy metals such as lead and concretes have often been used so far as such radiation-shielding materials. Lead is harmful upon absorbed in human bodies, and must be handled pursuant to strict regulations prescribed in the Ordinance on Prevention of Lead Poisoning. Lead must also be disposed under the category of specific hazardous industrial waste while mounting environmental, cost and other problems are taken into consideration.
As disclosed in Patent Publications 1 and 2 and described in Non-Patent Publication 1, concrete must have an increased thickness so as to obtain high shielding rates. Although concrete does not give rise to any problem with fixed structures, yet there are some grave problems with vessels of drum can or container size used for storage and transportation of radioactive materials: the amount of radioactive materials filled up in them is limited, and the weight of vessels themselves is too heavy for delivery. In addition, concrete may often crack with the progress of hydration reactions, resulting possibly in a leakage of radioactive material.
On the other hand, iron having a density of 7.8 g/cm3 is lighter in weight than lead having a density of 11.3 g/cm3; in order to obtain a shielding rate of 90% for Cs 137 γ-rays, however, at least 75 mm thickness is required as compared with 25 mm thickness for lead or the weight is twice as large as lead. Iron cannot thus provide any effective shielding material.
As disclosed in Patent Publications 3, 4, 5 and 6, heavy metals such as tungsten, an alternative to lead, are often kneaded with resins for sheeting. For instance, heavy metals may be milled with thermoplastic resins using a machine having strong shearing force such as a Banbury mixer or a kneader, after which the milled material is extruded through an extruding machine into a sheet product. This radiation-shielding material sheet may be cut depending on the configuration of an application site for use.
As disclosed in Patent Publication 7, a solvent may be used to make the liquid viscosity of a shielding material low enough for cast molding. However, Patent Publication 7 shows that the obtained sheet or molded product has a thickness of 5 mm at most, and says that such sheets may be put one upon another for greater thicknesses. A greater number of sheets will have limited use because of difficulty in retention of shape.
Further, the radiation-shielding rate is greatly affected by the density of the shielding material; so Patent Publications 3 and 4 teach that the packing fraction of heavy metals, etc. is low and, hence, the density of the shielding material is low, too, resulting in a low X-ray shielding rate. However, Patent Publications 5, 6 and 7 show that the packing fraction of heavy metals is increased up to 90% by weight or greater to increase the density of the shielding material up to that of lead, thereby making sure of the shielding rate comparable to that of lead.